Osteosarcoma is easily the most common bone tumor that affects children and youthful adults. Despite advances in using combination chemotherapy regimens, reaction to neoadjuvant chemotherapy in osteosarcoma remains a vital determinant of patient outcome. Lately, highly potent small molecule inhibitors of canonical Wnt signaling with the poly(ADP-ribose) polymerase (PARP)-family enzymes, tankyrases 1 & 2 (Tnks1/2), happen to be regarded as possible chemotherapy sensitizing agents. The aim of this research was to look for the ability from the highly specific Tnks1/2 inhibitor IWR-1-endo to sensitize chemotherapy-resistant osteosarcoma to doxorubicin. We discovered that IWR-1-endo considerably inhibited cellular efflux, as measured by cellular retention of Calcein AM and doxorubicin. Inside a type of doxorubicin resistant osteosarcoma, pre-treatment with IWR-1-endo strongly sensitized to doxorubicin. This sensitization reduced the doxorubicin IC50 in doxorubicin-resistant cells, although not in chemotherapy na?ve cells and caused doxorubicin-treated cells to amass in the G2/M checkpoint. Further, we discovered that sensitization with IWR-1-endo created elevated |?H2AX foci formation, indicating elevated DNA damage by doxorubicin. Taken together, our findings reveal that IWR-1-endo increases cellular responses to doxorubicin, by blocking efflux transport inside a drug-resistant type of osteosarcoma.